Light offers many advantages when used as a medium for propagating information, the foremost of which are increased speed and bandwidth. In comparison with electrical signals, signals transmitted optically can be switched and modulated faster and can include an even greater number of separate channels multiplexed together. For these, as well as other reasons, light wave transmission along optical fibers is widespread.
Light can be propagated through planar waveguide structures as well as optical fibers. Planar waveguide structures having a wide variety of functionalities are currently available and many new such devices and components will likely result from future research and development. These planar structures are advantageous because they can be compactly incorporated together in or on a planar platform, i.e. substrate, to form planar packages analogous to integrated circuits (ICs). These structures in general are referred to as integrated optics. Integrated optical “chips” comprise a substrate on which or in which various integrated optical components or devices are formed. Planar waveguides analogous to conductor traces in semiconductor electronic ICs that are mounted in or on the substrate are employed to guide light to various optical, electro-optical, and optoelectronic devices or components on the chip.
In many applications, it is desirable that the optical signal being transmitted through the planar waveguide structure be optically coupled into or out of the integrated optical chip. These signals may, for example, be coupled to an optical fiber that is oriented out of the plane, i.e., above or below, the planar waveguide structure via a grating coupler. The grating coupler, forming a part of the planar waveguide structure, may have a plurality of scattering elements designed to scatter light (or, equivalently couple light) along a predetermined optical path, where the scattered light has preferably a Gaussian intensity distribution.
The scattering elements have at least one characteristic, such as width, height and local index of refraction, which varies in magnitude among at least some of the scattering elements. The magnitude of the characteristic controls at least in part the scatter cross-sections of the scattering elements. The scatter cross-section is a quantity that determines how much of the incident light is scattered by a scattering element into a specific angle. Generally, in grating couplers where the scattering elements induce relatively weak scattering of light, the scatter cross-section increases as the size of the scattering element increases. However, when the grating coupler induces stronger scattering of light, the scatter cross-section often times oscillates as a function of the size of the scattering element. One possible reason for such oscillation is due to the interference of reflected light from the various interfaces associated with the grating coupler.
In addition to the oscillatory behavior, the scatter cross-section of an elongate scattering element may approach its lower limit as the size of the elongate scattering element decreases. The lower limit stems from the lithographic limit of the elongate scattering element, i.e., it is impractical to fabricate elongate scattering elements smaller than what lithographic processes can provide. Thus, there is a need for a systematic approach to configure the scattering elements considering the oscillatory behavior and lithographic limit such that the scatter cross-sections of the scattering elements can be arranged to couple light having an intended beam shape, preferably a Gaussian intensity distribution.